1. Field of the Invention
The present invention relates to a direct fuel injection type spark ignition internal combustion engine in which fuel is directly injected into a cylinder of the engine onto a surface of a recessed dish portion formed in a top portion of the piston. The fuel adhering to the surface of the dish portion is evaporated to generate a combustible gas which is ignited by a spark plug.
2. Description of the Related Art
Direct fuel injection type spark ignition internal combustion engines are disclosed, for example, in Japanese Patent Publications SHO 62-82222, SHO 62-17339, and SHO 62-659; and Japanese Utility Model Publications SHO 61-173728, and SHO 54-81005. Among them, Japanese Patent Publication SHO 62-82222 discloses a locational relationship between ports and a spark plug or a fuel injection nozzle. Japanese Utility Model Publication SHO 61-173728 discloses a piston/cylinder arrangement with a shallow recessed dish portion and a deep recessed dish portion formed in a top portion of a piston. FIGS. 15 and 16 illustrate the engine of Japanese Patent Publication SHO 62-82222 except the reference numerals; and FIG. 17 illustrates the piston of the engine of Japanese Utility Model Publication SHO 61-173728 except the reference numerals.
In the engine illustrated in FIGS. 15 and 16, a spark plug 122, which is difficult to be designed compact in size, extends between two exhaust valves 108 and 109, and a lower end portion of spark plug 122 is located close to a center of a combustion chamber. A fuel injection nozzle 123, which is comparatively easy to be designed small in size, extends between two intake valves 106 and 107, and a lower end portion of fuel injection nozzle 123 is located close to the center of the combustion chamber so as to oppose the lower end portion of spark plug 122. This arrangement prevents interference in location between spark plug 122 and fuel injection nozzle 123. Further, because the lower end portions of the spark plug 122 and the fuel injection nozzle 123 are located close to each other and oppose each other, fuel injected from fuel injection nozzle 123 directly collides with spark plug 122 so that prompt ignition is performed
In the engine illustrated in FIG. 17, a combustion chamber 220 includes a shallow dish portion 221 and a deep dish portion 222 both formed in a top portion of the piston. Unlike the present invention, shallow and deep dish portions 221 and 222 of the engine of FIG. 17 are arranged at a radially central portion of the piston so that the dish portions 221 and 222 and the piston have their axes in common. A spark plug 205 is greatly offset in a radial direction from the axis of the piston.
However, as will be illustrated below, some problems exist in the prior art engines.
With respect to the engine of FIGS. 15 and 16, in the case where the amount of fuel to be injected is determined so that optimum combustion is obtained in a low engine load operation, when a greater amount of fuel is supplied in a high engine load operation than the amount determined on the basis of the low engine load operation, the fuel will not be perfectly burned and the non-burned fuel is likely to cause smoking in the vicinity of spark plug 122 and also in the exhaust gas. On the contrary, in the case where the amount of fuel to be injected is determined so that optimum combustion is obtained in a high engine load operation, when less fuel is supplied in a low engine load operation than the amount determined on the basis of the high engine load operation, the mixed gas of fuel and air will be too lean to be smoothly burned, and it is difficult to form a rich gas layer around spark plug 122. In other words, in this engine, when the ignition characteristic is satisfied at low engine loads, the combustion characteristic corresponding to a power characteristic of the engine can not be satisfied; and when the power characteristic is satisfied at high engine loads, the ignition characteristic of the engine can not be satisfied. This means that the ignition characteristic at low engine loads and the power characteristic at high engine loads are not compatible with each other. In addition, because spark plug 122 is located between the two exhaust ports 112 and 113, the exhaust ports 112 and 113 have to be bent in the plan view thereof to provide therebetween a space for mounting the spark plug 122. The bent exhaust ports 112 and 113 will increase flow resistance of the exhaust gas and will decrease the engine power.
With respect to the engine of FIG. 17, the spark plug 205 is located far from the axis of the combustion chamber on the opposite side of the fuel injection nozzle with respect to the axis of the combustion chamber, thereby increasing a flame propagation distance (defined as a distance from spark plug 205 to the fuel injection nozzle). As a result, the combustion characteristic of the engine will drop in comparison with that of an engine with a short flame propagation distance. Further, because the engine of FIG. 17 does not illustrate the relationship between ports and spark plug 205 or the fuel injection nozzle, even if the arrangement of FIG. 17 were attempted to be applied to a multiport engine, for example, an engine with two intake ports and one exhaust port, it would be very difficult to arrange the fuel injection nozzle and the spark plug in the limited space between the ports of such a multiport engine.